Vapour compression systems normally comprise a compressor, a heat rejecting heat exchanger, such as a condenser or a gas cooler, an expansion device and an evaporator arranged in a refrigerant path. A flow of refrigerant circulates the system and is alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, substantially gaseous refrigerant is compressed in the compressor. The compressed refrigerant is supplied to the condenser/gas cooler. In the case of a condenser, the refrigerant condenses, i.e. refrigerant leaving the condenser is in a substantially liquid form. The condensed refrigerant is then expanded in the expansion device before entering the evaporator, where it evaporates. The refrigerant leaving the evaporator is supplied to the compressor, and the cycle is repeated. The supply of refrigerant to the evaporator is often controlled on the basis of a measured superheat value of the refrigerant leaving the evaporator, and in order to obtain a small, but positive, superheat. Thereby it is obtained that the refrigeration capacity of the evaporator is utilised to the greatest possible extent, without risking that liquid refrigerant passes through the evaporator.
In some vapour compression systems the evaporator comprises two or more evaporator paths arranged fluidly in parallel along the refrigerant path. This is, e.g., the case if the vapour compression system comprises two or more evaporators arranged fluidly in parallel, or if the evaporator comprises two or more evaporator tubes arranged fluidly in parallel within the evaporator. In this case the available refrigerant must be distributed among the evaporator paths. It is desirable to perform this distribution in a manner which ensures that the entire refrigeration capacity of the evaporator or evaporators is utilised to the greatest possible extent, while avoiding that liquid refrigerant leaves the evaporator(s).
One way of controlling the operation of a vapour compression system comprising at least two evaporator paths is disclosed in WO 2008/151630 A1. In the method disclosed in WO 2008/151630 A1 the available refrigerant is distributed among two or more evaporators in accordance with an obtained distribution key. The amount of available refrigerant is controlled in response to a superheat value of refrigerant leaving the evaporators, the value being measured at a common outlet of the evaporators, and in order to obtain an optimum superheat value. The distribution key may be generated in such a manner that due consideration is taken to special operating conditions of each of the evaporators in order to obtain optimal filling for all of the evaporators. It may be possible to adjust the distribution key during operation, or the distribution key may be fixed initially.
One drawback of the method disclosed in WO 2008/151630 A1 is that it is not always possible to reach an optimal distribution key within a reasonable time. This is in particular a disadvantage in the case that the operating conditions of the vapour compression system are subject to changes which are substantial and/or rapid. In this case the vapour compression system may be operated in a manner which does not allow for optimal utilisation of the potential refrigeration capacity of the evaporator for a substantial part of the operating time.